The present invention relates to a liquid crystal display device.
The conventional liquid crystal display device has a display screen comprising liquid crystal elements arranged on a plane and a pair of front and back substrates sandwiching the liquid crystal elements. Each of the front and back substrates are placed an orientational film having a rubbing layer of a predetermined direction, a transparent insulating substrate with a plurality of transparent electrodes arranged in lines on it, and a polarizing plate with a polarized light absorbing axis oriented in the predetermined direction, in this order. The front and back substrates face each other with the liquid crystal elements between them so that the orientational films are kept in contact with the front and back sides, respectively, of the liquid crystal elements. Generally, the lines of the transparent electrodes on the front substrate intersect the lines of the transparent electrodes on the back substrate, forming a lattice. Consequently, the display screen comprises an effective display region containing picture elements in the same quantity as the number of intersections M.times.N (M, N=the number of transparent electrode lines) between the transparent electrode lines, and a region other than the effective display region, called a non-display region where transparent electrodes exist on either the front or the back substrate but not on both substrates. Normally, the effective display region is framed by the non-display region on the display screen.
The orientational films and the polarizing plates are placed on both sides of the liquid crystal elements in such a manner that the rubbing layer on one side of the substrater forms a predetermined angle with the rubbing layer on the other side and that the polarized light absorbing axis of the polarizing plate on one side of the substrates is formed a predetermined angle with the direction of the polarized light absorbing axis of the polarizing plate on the other side. These angles determine the type of the display screen, that is the type of the screen can be either a black type (or normally dark type) in which the screen transmits light when voltage is applied between the electrodes and shuts off light when voltage is not applied, or a white type (or normally bright type) in which the screen transmits light when voltage is not applied between the electrodes and shuts off light when voltage is applied. Either type of the screens can be employed in conventional liquid crystal display devices.
In the display screen of the above liquid crystal display device, electrodes exist only on one side or the other of the liquid crystal elements in the non-display region, so that a driving voltage is not applied to the electrodes in the non-display region. Accordingly, when the display device is in the active state, the non-display region is either black or white depending upon the screen type. Specifically, on a normally black type screen, the non-display region is distinguished as a black frame, whereas on a normally white type screen, the non-display region is not distinguishable from the effective display region.
Therefore, when characters are displayed on the normally black type screen, a part of the characters may combine with the black non-display region, resulting in an illegible display of characters. When characters are to be displayed on the normally white type screen, on the other hand, it is difficult for the operator to plan the layout because the effective display region cannot be readily distinguished from the non-display region; the operator is obliged to input characters without clear knowledge of the boundary of the effective region. Against this background, a liquid crystal display device whose non-display region can be made dark or bright as desired depending upon the state on the effective display region has been desired but not yet realized.